Roller electrodes of the type referred to usually carry large electrical currents generating considerable heat. It is therefore necessary to provide them with channels close to the peripheral surface for the circulation of a cooling fluid. Moreover, a significant part of the roller must consist of a metal or alloy of good electrical conductivity in order to provide a low-resistance path between the contact surface of the roller and the associated current supply.
Conventionally, low-resistance material is deposited as a coating on the periphery of a cylindrical roller body of steel or cast iron which is provided with axially extending bores near that periphery for the circulation of the cooling fluid. In a device of this type described in German printed specification No. 1,771,277, for example, the bores constituting the cooling channels are lined with a corrosion-resistant substance and are closed at the ends by covers of conductive sheet metal. The resulting structure is electrolytically coated with a metallic skin consisting of a highly conductive intermediate layer, e.g. of copper, and an outer layer of hard conductive metal such as chromium.
The electrolytic coating of the roller body is time-consuming and correspondingly expensive. The chromium-coated copper layer is prone to wear and is readily attacked by the electrolyte through the rather brittle chromium layer. This may cause local flaking of the chromium, even after relatively short periods of operation; as a result, the treated workpieces--e.g. galvanized or tin-coated webs of sheet steel--are of lower quality. The roller, therefore, requires frequent polishing and possible recoating which is again an expensive procedure. Moreover, the relatively thin chromium layer is also susceptible to mechanical damage by heavier workpieces resting thereon, with development of tiny cracks facilitating penetration of the electrolyte with resulting partial destruction of the copper substrate. The large currents tending to traverse these defective areas may result in scorching of the roller body itself which then becomes unusable.